I mean the market literally is the average of everybody's position so I would say it's a tautology that the average performance of everyone who participates in a market will equal an index of that market.

But sure you could always define efficiency in an unattainable way by saying no matter how much you beat the market your streak could end down the line and the limited sample size means it doesn't prove anything.

And to me that's what efficient markets are. This unfalsifiable word game you can play where as long as the definition is flexible enough, you get to say the sentence "markets are efficient".